Selective solvent extraction process

ABSTRACT

Aromatic hydrocarbons of increased purity are obtained in a selective solvent extraction process comprising an extraction step followed by one or more distillation steps by introducing a low boiling hydrocarbon fraction into at least one of the distillation steps. Optionally, the low boiling hydrocarbon fraction may be obtained from the feed mixture to the process.

United States Patent 1191 Van Kleef et al. 1 Feb. 25, 1975 [54] SELECTIVE SOLVENT EXTRACTION 3,294,679 12/1966 Donovan 260/674 SE PROCESS 3,385,783 5/1968 'Okuma 260/674 s13 3,396,101 8/1968 Broughton 208/313 [75] Inventors: Alfred L. Van Kleef; Derk R tfel; 3,433,849 3/1969 Eisenlohr 260/674 313 Frits W. Van Vollenhoven, all of 3,468,792 9/1969 Uitti et a1 1 260/674 SE The Hague, Netherlands 3,590,092 6/1971 Uitti et a1. 260/674 SE [73] Assignee: Shell Oil Company, New York, NY. FOREIGN PATENTS OR APPLICATIONS 22 Filed; Sept. 8 7 1,514,023 2/1968 France 208/313 717,725 11/1954 Great Britain 260/674 SE [21] Appl. No.: 70,192

I Pr imary ExaminerA. Louis Monacell [30] Foreign Application Priority Data Assistant Examinerl :rank Sever Sept. 26, 1969 Netherlands 6914611 Attorney Agen" [52] US. Cl. 203/58, 260/674 SE [57] ABSTRACT [51] Int. Cl B01d 3/34 Aromatic h ydrocarbons of increased punty are ob- [58] held of Search 208/313 5 tained in a selective solvent extraction process comprising an extraction step followed by one or more distillation steps by introducing a low boiling hydrocar- [56] References cued bon fraction into at least one of the distillation steps. UNITED STATES PATENTS Optionally, the low boiling hydrocarbon fraction may 2,434,322 H1948 Latchum 203/43 be obtained from the feed mixture to the process. 2,461,346 2/1949 Patterson..... 203/43 2,809,925 10/1957 Nelson 203/46 6 Clams, 2 Drawing Flgures RAFFINATE NON-AROMATICS AROMATICS ll LIGHT HYDROCARBON SOLVENT V P DlSTlLLATlON DlSTlLLATlON 3 8 COLUMN COLUMN EXTRACTOR 2 I32 FEED 2 EXTRACT PHASE Z J 5 SOLVENT; 9

PAIENTEUFEB25|975Q 3,858,310

RAFFINATE NbN-AROMATICS AROMATICS 1 LIGHT 7 HYDROCAQRBON SOLVENT I 5 'DISTILLIATION DISTILLATION 3 a COLUMN 'YCOLUMN YEXTRACTOR v I :3 FEED] EXTRACT" PHASE) 7 l f 5 SOLVENT; 9

- FIIG. I LOW BOILING HYDRQCARBONSjf '28 IQR'AFFII'NATE v FRACTIONATOR SOLVENT FEED HIGH BOILING KEXTRA C-i' HYDROCARBONS I PHASE SOLVENT) F re. 2

- THEIR ATT-OR'NEY SELECTIVE SOLVENT EXTRACTION PROCESS This invention relates to an improved process for separating mixtures of hydrocarbons. More particularly, it relates to a liquid-liquid extraction process wherein aromatic hydrocarbons are separated from non-aromatic hydrocarbons by means of a selective solvent which boils at a higher temperature than feed mixture to be separated.

Processes for separating aromatic hydrocarbons from non-aromatic hydrocarbonsby the use of selective solvents are well known in the-art. In a typical process a feed stream containing a mixture of aromatic hydrocarbons and non-aromatic hydrocarbons is introduced into an extraction zone in which it is countercurrently contacted with a solvent selective for aromatic hydrocarbons, thereby forming a raffinate phase containing a larger concentration of non-aromatic compounds, and an extract phase containing a larger concentration of aromatic hydrocarbons. The dissolved aromatics are subsequently recovered from the extract phase by one or more distillation steps.

In order to ensure that the recovered components have a sufficient degree of purity, special measures are frequently required such as subjecting the extract phase (or a material obtained therefrom) to an extractive distillation using very large quantities of the extraction agent. A number of other methods have been proposed for increasing the purity of the recovered components, but by and large, such special measures have proved economically unattractive. Therefore, it is evident that an improved process for increasing the purity of recovered aromatic components, which is adaptable to currently employed equipment and hence does not require extensive capital outlays, would be highly desirable.

It has now been found that the recovery of aromatic hydrocarbons in a pure state is highly promoted by introducing a low boiling hydrocarbon fraction into one or more distillation steps of a solvent extraction process. in accordance with the invention, a hydrocarbon feed mixture containing aromatic and non-aromatic hydrocarbons is subjected to a liquid-liquid extraction with a solvent which boils higher than the starting mixture and is selective for aromatic hydrocarbons, thereby obtaining an extract phase enriched in the aromatic components and a raffinate phase enriched in non-aromatic components. The dissolved aromatic components are recovered from the extract phase by using one or more distillation steps from which at least one stream containing non-aromatics is obtained. The recovery of aromatics in such a distillation step or steps is promoted by the introduction of a low boiling hydrocarbon mixture, e.g., a C to C fraction, containing preferably both aromatic and non-aromatic hydrocarbons into the distillation zone. It has been found that the introduction of such a'low boiling hydrocarbon fraction has a remarkable effect on the purity of the aromatic hydrocarbons ultimately recovered. That this effect is not, for example, due to a simple stripping is evident from the fact that it is not necessary to introduce the light hydrocarbon mixture into the bottom part of the distillation column as is generally required for stripping. On the contrary, highly desirable results are obtained when the mixture is introduced at the middle or top of the distillation column.

The boiling ranges of lower boiling fraction and starting feed mixture may overlap. That is, the lower boiling mixture may comprise components having a higher boiling point than certain components of the starting mixture. In order to recover aromatic hydrocarbons in a highly pure state, it is preferred that the highest boiling, non-aromatic hydrocarbon of the lower boiling mixture, boil not more than 100C higher than the lowest boiling aromatic hydrocarbon in the starting mixture. This means that the higher boiling non-aromatic hydrocarbon of the lower boiling mixture either itself boils lower than the lowest boiling aromatic hydrocarbon to be recovered in the starting mixture or boils equally high, or at most C higher. In this connection, aromatic hydrocarbon components to be recovered are understood to mean those components which it is desired to recover in the purest possible state. Other aromatic or non-aromatic hydrocarbon components, the recovery of which in a highly pure state is not particularly valued, can, of course, be present in the starting mixture.

. The purity of recovered components is also favorably influenced if the lower boiling hydrocarbon mixture is introduced into the distillation step at least in part below the level at which extract phase (or a material recovered therefrom in one or more previous steps) is supplied to this step. Although less desirable, introduction of the lower boiling hydrocarbon at the same or at a higher level is also possible. Alternatively, this introduction may also be effected at more than one point of the distillation step, it being possible to use either the same lower boiling mixture at each point, or two or more different mixtures of this type. In addition, it is possible to introduce a lower boiling mixture into a single distillation step or into more than one distillation step, provided such a distillation step meets the requirement that at least one stream containing nonaromatic hydrocarbon components can be obtained therefrom. lf lower boiling mixture is introduced into more than one distillation step, the same lower boiling mixture, or two or more different mixtures of this type, may be used. The stream of streams recovered from one or more of such distillation steps may contain the non-aromatic hydrocarbon components either substantially free from selective solvent and/or from more readily soluble components, or in admixture with one of these two substances. The optimum composition of these stream or streams in any particular case will depend on the means selected for the processing of the extract phase (type of distillation steps used. the way they are arranged in respect to each other, etc.).

In a distillation step into which lower boiling mixture is introduced, not only the recovery of components originating from the starting mixture takes place, but also the separation of introduced lower boiling mixture itself into non-aromatic and aromatic components or into streams containing such components. As previously pointed out, this is the reason why a distillation step into which lower boiling mixture is introduced, should be one from which at least one stream containing non-aromatic components is normally recovered. Components of this type originating from the lower boiling mixture thus can be incorporated into the nonaromatic hydrocarbon stream which prevents them from finding their way into, and thus contaminating, the aromatic hydrocarbon components.

In order to increase the effectiveness of the separation of the lower boiling mixture into aromatic and non-aromatic components, and to maximize recovery lower boiling mixture, and/or separately; at a level that is at least ashigh as the point at which the aforementioned streams are introduced. It is particularly preferred that the additional solvent be introduced into the distillation steps at a higher level than that of the lower boiling mixture.

The non-aromatics containing stream from the distillation step or steps as mentioned above, is preferably recycled, at least in part, to the liquid-liquid extraction as backwash". The recycling of this stream has been found to favorably promote the separation of the starting mixture by extraction, just as lower boiling mixture itself promotes the recovery in a pure state of the dissolved aromatics from the extract phase.

The extraction and distillation steps can be carried out using any suitable equipment known in the art. The

extraction can be very desirably effected with the aid of a rotating disc contactor such as described, for example, in UK. Pat. specification No. 659,241, while distillation is conveniently carried out with the aid of a column provided with valve trays or trays with preferential discharge (such as described in U.K. Pat. specification Nos. 591,929 and 764,640, respectively). Distillation may be effected in one or more of the steps present, by expansion, if desired.

'ln principal, any mixture of lower boiling hydrocarbons having the boiling point relationship of the starting mixture he'reinbefore outlined is suitable for use in accordance with the invention. It is especially preferred to use at least one lower boiling mixture which has been obtained at least in part from an original material from which the feed mixture has also been obtained.

Thus, in a preferred embodiment of the invention, the original starting mixture (e.g., a C to C mixture of. aromatic and non-aromatic hydrocarbons) is split into two or more fractions, the higher boiling of which (e.g., C to C is used as feed to the extraction step, while one or more of the lower boiling fractions (e.g.,

C to C is employed, either separately or in admixture with another low boiling hydrocarbon fraction, in one or more of the distillation steps as previously discussed. The separation of the starting mixture may be conveniently accomplished by distillation or other suitable fractionation means, e.g., extractive distillation.

By proceeding in this manner, it has been found that significant economies are achieved in the extraction step, since only a portion of the original starting mixture is being extracted as a result of which equipment and utilities requirements are reduced. Hence, the desired low boiling hydrocarbon fraction employed in the distillation step(s) to increase the purity of the recovered product obtained, as it were, in the bargain thus making this embodiment of the invention particularly attractive.

Suitable starting mixtures and/or lower boiling mixtures include straight-run hydrocarbon fractions, or hydrocarbon fractions'obtained from a conversion process such as thermal cracking, catalytic cracking and- [or hydrocracking, including gasolines, kerosenes or gas oils. Particularly suitable starting mixtures and/or lower boiling mixtures include normally liquid, catalytically reformed hydrocarbon fractions, such as gaso lines, and/or such fractions obtained from a pyrolysis process. The latter type of fraction is preferably first refined, for example, by using a catalytic treatment with hydrogen or a hydrogen-containing mixture.

In the process according to the invention a variety of selective solvents may be used, for example, furfural, phenol, acetonitrile, sulphoxide (such as dimethyl sulphoxide), fattyacid alkyl amides (such as dimethyl formamide). Especially preferred solvents are those of the sulfolane, glycol, and/or the pyrrolidone/piperidone type; i.e., respectively: sulfolane (cyclic tetramethylene sulphone), the unsaturated analogues thereof and the derivatives of both, as described, for example in U.K. Pat. specification No. 625,505; lower polyalkylene glycols (such as di-ethylene, tetraethylene and dipropylene glycol); pyrrolidone and piperidone, as well as their derivatives (such as N-alkyl, in particular N-methyl pyrrolidone and piperidone). If desired, the selective solvent may contain a quantity of a substance, such as water, which has a favorable effect on the selectivity and/or the solvent power thereof. Of the aforementioned selective solvents, sulfolane has been found to be particularly advantageous.

The invention will be further illustrated by reference to the drawing.

In the embodiment shown in FIG. 1 of this drawing, a feed mixture is supplied through line 1 to extractor 2, in this case at one point and in the lower part of the column. If desired, the feed may be supplied at more than one point and at higher levels.

In extractor 2, the starting mixture is subjected to ex traction at a suitable temperature and pressure, if desired with the use ofa temperature gradient. Fresh and- /or recycled solvent is supplied through line 3, in this case separately and at one point, but possibly also together with the starting mixture and/or through more than one inlet (not shown).

If desired, non-aromatic components (possibly admixed with aromatic components), originating from the process itself or not, may be introduced as backwash into the lower part of extractor 2 (not shown).

Raffinate is withdrawn from column 2 through line 4. If desired, this raffinate may be subjected to a further treatment, for example, a washing treatment with a suitable washing agent (such as water) to remove any solvent present (not shown). An extract phase containing both aromatic and non-aromatic components and solvent, is passed through line 5 to distillation column 6 (if desired, operated under reflux not shown and/or with reboiling). From column 6, a stream containing primarily nonaromatics with lesser amounts, if any, of solvent, water or aromatics is withdrawn through line 7. In this case, column 6 is the only distillation step from which such a stream is withdrawn. However, it is understood that there can be more of such distillation steps and/or streams, possibly differing in composition from the stream mentioned.

According to the invention, a mixture containing aromatic and non-aromatic hydrocarbons boiling lower than the starting mixture and either of the same type, at least in part, or not of the same type as those of the starting mixture, is introduced into column 6 through line 8. in this case only one such mixture is supplied and the supply is'effected together with that of the extract phase. However, it is also possible to effectthe supply at another point and/or at several points, employing the same lower-boiling mixture, or, at least to some extent, mutually different lower boiling mixtures.

The non-aromatic hydrocarbons originating from the lower boiling mixture together with the non-aromatics originating from the starting mixture are withdrawn in vapor phase from column 6, through line 7. In this case, column 6 is operated in such a manner that the material withdrawn from the bottom part contains selective solvent and dissolved aromatic hydrocarbons, with little or no non-aromatic hydrocarbons. The material from the bottom of column 6 is passed through line 9 to distillation column 10 wherein it is separated into solvent and aromatic hydrocarbons, the latter being discharged through line 11 as a single stream containing aromatic components originating both from the starting mixture and from the lower boiling mixture. There can, however, be more of such streams, which possibly contain either only or mainly components originating from the starting mixture, or only or mainly components from the lower boiling mixture. In addition, the stream containing the aromatic components can, if desired, be subjected to further separation, for example, by distillation into one or more fractions, by treatment with a washing agent or solvent to obtain the individual components, or groups of components which differ slightly from each other in nature. Further distillation may also be employed to remove the very last traces of solvent and/or non-aromatic hydrocarbons (not shown). The selective solvent is withdrawn from column l0 and in this case recycled wholly or partly to column 2, for use in extraction. Alternatively, this solvent may be recycled, at least in part, to column 6 and- /or withdrawn, for example, to a purifying plant (not shown).

Instead of two separate columns 6 and 10, a system similar to that described in UK. Pat. specification No. 717,725 or No. 980,973, may also be employed in which the extraction and distillation columns are combined to form one column, and the material which in the case shown is discharged through line 11, is withdrawn as a sidestream from the combined column in question. In the present case, column 10 is operated under reflux (not shown) and with the aid ofa stripping medium, such as steam, supplied through a line 12. A portion of the material flashed in the reboiler associated with column 6 is also used as stripping medium in column 10, and is introduced into it at a somewhat higher level than the stripping steam through line 13.

Another embodiment of the process according to the invention is shown in FIG. 2. According to this preferred embodiment, the lower boiling mixture supplied through line 28, as well as the feed mixture supplied through line 16, originate (in this case entirely but possibly also in part) from one and the same original material supplied through line 14. In this case, the lower boiling mixture and feed mixture have been prepared from the original material, as top product and bottom product, respectively, by simple distillation in fractionating column 15. It is also possible, however, to prepare these mixtures from the original material (if desired, in

part) by, for example, extractive distillation. It is alsopossible to carry out distillation in more than one column and/or with obtainment of more than one fraction, all of which, or only part of which, may be used as the feed mixture or the lower boiling mixture, respectively.

The high boiling hydrocarbon fraction is withdrawn from column 15 and is introduced as feed to extractor 17 wherein it is subjected to extraction as hereinbefore discussed.

Raffinate, containing a larger proportion of nonaromatics than the feed mixture, is withdrawn from the extraction column through line 26, while the extract phase containing a greater proportion of aromatics than the feed mixture is withdrawn through line 18 and is passed to distillation column 19 together with additional solvent via line 25.

The lower boiling hydrocarbon material withdrawn from column 15 is introduced into column 19 via line 28 at a point lower than the level of the extract phase and additional solvent, which are supplied through line 18, in accordance with the preferred embodiment of the invention. While the additional solvent in this instance is introduced into column 19 together with the extract phase, it is also possible to effect the introduction of the solvent with the lower boiling mixture instead, or alternatively partly with the extract phase and partly with the lower boiling hydrocarbon fraction.

In the embodiment shown, the overhead stream (line 20) from column 19 is partly recycled as backwash to a lower part of column 17. If desired, the overhead stream may be condensed prior to recycle and/or subjected to phase separation to separate the phase containing solvent and/or, for example, water (not shown). While the recycling in this case is effected at one point in column 17, it is understood that recycling may be effected at several points, or may be omitted entirely, or effected employing a non-aromatic stream originating from elsewhere than column 17.

The stream withdrawn from the bottom of column 19 (line 21) containing primarily solvent and aromatic hydrocarbons, with lesser amounts of water and/or nonaromatics, is passed to distillation column 22 wherein it the aromatic hydrocarbons are separated and recovered as the overhead stream via line 23. Stripping stream as required is introduced into column 22 via line 24. The stripping medium might also originate from the reboiler of column 19 which for simplicitys sake is not shown.

In the embodiment shown, the additional solvent is not substantially pure but instead contains a minor quantity of dissolved aromatic components. While the additional solvent added to column 19 may be substantially pure, it is preferred that a solvent stream contain a small concentration of aromatics be employed as described in copending U.S. Pat. application Ser. No. 31,981 filed Apr. 27, 1970, now abandoned.

The process of the invention will be further illustrated by means of the following example.

EXAMPLE processed by means of equipment as shown in FIG. 2,

in a quantity of 1,000 t/d. The material in question consisted partly of catalytically reformed gasoline, and partly of gasoline from a pyrolysis process which had subsequently been hydrorefined.

tained 91.3 percent by weight of aromatics (including 9.4 percent by weight of benzene, 31.7 percent by weight of toluene and 39.7 percent by weight of'xylene), 6.5 percent by weight of paraffins and 2.2 percent by weight of naphthenes. The lower boiling mixture introduced into column 19 contained 69.3 percent by weight of aromatics (including 52.7 percent by weight of benzene, 16.0 percent by weight of toluene and 0.6 percent by weight of xylene), 24.5 percent by weight of paraffms and 6.2 percent by weight of naphthenes.

In column 17 the starting mixture was subjected to an extraction with the aid of 2,120 t/d of sulfolane supplied through line 27. A quantity of 212 t/d of raftinate was obtained through line 26, containing 70.6 percent by weight of paraffins, 19.2 percent by weight of naphthenes and only 10.2 percent by weight of aromatics.

A quantity of 3,106 t/d of extract phase was withdrawn from the bottom of column 1.7 through line 18 and subsequently passed to column 19. In addition to the above-mentioned 469 t/d of lower boiling fraction,

a quantity of 3,200 t/d of a sulfolane-rich stream containing 0.7 by weightof hydrocarbons was also introduced into column 19 through line 18 via line 25. C01- umn 19 was operated at a pressure at 1.2 atm. abs., with a top temperature of 1 13C and a bottom temperature of 145C.

From the .upper part of column 19 a quantity of 666 t/d of a hydrocarbon fraction was withdrawn through line.20, which fraction was recycled as a whole to column 17. From the lower part of column 19, a quantity of 6,109 t/d of a fraction containing sulfolane and aromatic hydrocarbons and substantially no non-aromatic hydrocarbons, was withdrawn through line 21, the latter fraction being passed to column 22. Column 22 was operated at a pressure of 0.37 atm. abs., a top temperature of 166C, stripping stream being introduced at the bottom through line 24.

From the bottom of column 22 a quantity of 2,120 t/d of sulfolane was withdrawn and recycled through line 27 to column 17. At a point between the feed inlet and the bottom of column 22, the above-mentioned 3,200 t/d of sulfolane-rich stream were withdrawn through line 25 and recycled to column 19. From the upper part of column 22, a quantity of 788 t/d of extract was withdrawn through line 23, containing more than 99.9 percent by weight of aromatic hydrocarbons (including 37.6% by weight of benzene, 30.6 percent by weight of toluene and 25.9 percent by weight of xylene).

The extract was subsequently separated by distillation to give a benzene, a toluene and a xylene fraction;

these fractions contained as little as ,100, 180 and parts per million by weight of non-aromatic hydrocarboris, respectively.

For comparison, the process described was modified to the extent that no lowerboiling mixture was employed in column 19.'The same original material was directly introduced into extraction column 17 and subjected to extraction (with the same quantity of sulfolane), the same quantity of sulfolane-rich fraction again being supplied to column 19. In this case, the benzene, toluene and xylene fractions obtained from the extract contained 340, 1,360 and 2,770 parts by weight per million of non-aromatics, respectively, indicating that, by application of the process according to the invention, the purity ofbenzene, toluene and xylene is increased by a factor of approximately 3%, 7-% and 21, respectively;

We claim as our invention:

1. A process for separating aromatic hydrocarbons from a mixture of aromatic and non-aromatic hydrocarbons boiling in the C to C range which comprises (1) splitting said mixture by distillation into a higher boiling, C to C hydrocarbon fraction and a lower boiling C to C hydrocarbon fraction, (2) subjecting said higher boiling fraction to a liquid-liquid extraction with a solvent selective for aromatics to obtain an extract phase enriched in aromatic hydrocarbons and an raffinate phase enriched in non-aromatic hydrocarbons. (3) subjecting said extract phase to distillation in a first distillation zone into which said lower boiling hydrocarbon fraction and additional amounts of said selective solvent are simultaneously introduced, (4) withdrawing a stream further enriched in aromatic hydrocarbons from said first distillation zone and passing said stream into a second distillation zone wherein said selective solvent is separated from said aromatic hydrocarbons, and a 1 high purity aromatic hydrocarbon stream is recovered therefrom.

2. The process of claim 1 wherein the low boiling hydrocarbon fraction is introduced into the distillatin zone at a point lower than that at which the extract phase or the material obtained therefrom is introduced.

3. The process of claim 1 wherein the additional selective solvent introduced into the first distillation zone contains a minor amount of aromatic hydrocarbons and is obtained through recycle from the second distillation zone.

4. The process of claim 1 wherein the selective solvent is sulfolane.

5. The process of claim 4 wherein the hydrocarbon mixture to be separated comprises a catalytically reformed, normally liquid hydrocarbon fraction, or a refined, normally liquid fraction obtained from a pyrolysis process.

6. The process :of claim 4 wherein the lower boiling hydrocarbon introduced into the first distillation zone contains 69.3 percent by weight aromatics and 30.7

percent by weight non-aromatics. 

1. A PROCESS FOR SEPARATING AROMATIC HYDROCARBONS FROM A MIXTURE OF AROMATIC AND NON-AROMATIC HYDROCARBONS BOILING IN THE C5 TO C15 RANGE WHICH COMPRISES (1) SPLITTING SAID MIXTURE BY DISTILLATION INTO A HIGHER BOILING, C9 TO C15 HYDROCARBON FRACTION AND A LOWER BOILING C5 TO C8 HYDROCARBON FRACTION, (2) SUBJECTING SAID HIGHER BOILING FRACTION TO A LIQUID-LIQUID EXTRACTION WITH A SOLVENT SELECTIE FOR AROMTICS TO OBTAIN AN EXTRAT PHASE ENRICHED IN AROMATIC HYDROCATBONS AND AN RAFFINATE PHASE ENRICHED IN NON-AROMATIC HYDROCARBONS, (3) SUBJECTING SAID EXTRACT PHASE TO DISSTILLATION IN A FIRST DISTILLATION ZONE INTO WHICH SAID LOWER BOILING HYDROCARBON FRACTION AND ADDITIONAL AMOUNTS OF SAID SELECTIVE SOLVENT ARE SIMULTANEOUSLY INTRODUCED, (4) WITHDRAWING A STREAM FURTHER ENRICHED IN AROMATIC HYDROCARBONS FROM SAID FIRST DISTILLAYION ZONE AND PASSING SAID STREAM INTO A SECOND DISTULLATION ZONE WHEREIN SAID SELECTIVE SOLVENT IS SEPARATED FROM SAID AROMATIC HYDROCARBONS, AND A HIGH PURITY AROMATIC HYDROCARBON STREAM IS RECOVERED THEREFROM.
 2. The process of claim 1 wherein the low boiling hydrocarbon fraction is introduced into the distillatin zone at a point lOwer than that at which the extract phase or the material obtained therefrom is introduced.
 3. The process of claim 1 wherein the additional selective solvent introduced into the first distillation zone contains a minor amount of aromatic hydrocarbons and is obtained through recycle from the second distillation zone.
 4. The process of claim 1 wherein the selective solvent is sulfolane.
 5. The process of claim 4 wherein the hydrocarbon mixture to be separated comprises a catalytically reformed, normally liquid hydrocarbon fraction, or a refined, normally liquid fraction obtained from a pyrolysis process.
 6. The process of claim 4 wherein the lower boiling hydrocarbon introduced into the first distillation zone contains 69.3 percent by weight aromatics and 30.7 percent by weight non-aromatics. 